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Abstract

Abstract.
Epoxy acrylate has been widely used as optical resin for applications such as cladding, the core of a waveguide, and other photonic devices. In this study, sustainable resin from edible oil was used as an alternative to epoxy acrylate. Structural features and the transmission of planar thin-film resin from an ultraviolet-visible spectroscopy (UV-VIS) spectrometer were investigated upon UV exposure. It was found that high transmission still persists for all samples with and without an UV absorber for exposed and unexposed samples. The film was found to absorb strongly below 400 nm. A change in the cut-off wavelength was observed upon exposure. Thin-film hardness and its dynamic indentation in the load-unload mode with different test forces were evaluated. Vickers hardness and the elastic modulus were determined for unacrylated epoxidized soybean oil (ESO) and acrylated epoxidized soybean oil (AESO). It was found that the AESO has a higher Vickers hardness and elastic modulus than those of unacrylated thin film. The Vickers hardness and elastic modulus were found to increase as the applied test force increased. The refractive index, thickness, and modes present were characterized from a spin-coated planar thin film. The refractive index in the transverse electric mode (TE) and transverse magnetic mode (TM) were determined and compared for unacrylated and acrylated epoxidized oil.

Figures in this Article

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Introduction

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Ultraviolet (UV) curable polymer systems are used for high-performance coatings, adhesives, sealants, potting, and encapsulation. They include polyurethanes, epoxies, polyester, and acrylics. Many polymers can form “radical species” when exposed to ultraviolet light that generally result in cross-linking, increased molecular weight, insolubilization and a brittle film. Many in the industry have concentrated on this reaction to extend the life of plastic coating and components. The semiconductor industry has used this effect to its advantage to produce polymeric stencils, which are resistant to the acids and bases used to fabricate semiconductor devices and circuitry, and encapsulants for mechanical and corrosion protection of the chips.1

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There is increasing interest in the use of polymers for integrated optics. In the last decade, many components have been designed with remarkable properties.2,3 Significant progress has been made to make them reliable. Optical devices are classified based on their materials into inorganic devices and organic devices. Optical devices using organic materials have been recently highlighted due to their rapid response speed (<1fs), high nonlinear coefficient, and low cost. However, it has several drawbacks, such as an optical loss higher than that of inorganic devices and the need for a polishing process.4

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Photosensitivity in optical materials offers a great practical advantage for the design of families of passive and active optical devices for applications in communications, data storage, imaging, and sensing. Photosensitivity in its broadest sense refers to the optical response of a material that results in a refractive index modulation, produces birefringence, leads to a photorefractive effect, and produces defect centers and damage, among other effects.5,6

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Many advances in polymeric waveguide technologies were developed to fulfil the needs of the growing semiconductor industry. To perform as a waveguide, the polymer needs to have a higher refractive index than the substrate. The refractive index, thickness, birefringence, and propagation loss are important parameters that characterize films used for waveguide applications; thin films are normally measured with the m-line technique using prism coupling in both TE and TM field directions.7

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The controlled refractive index can be obtained by varying the spin speed and material formulations; thermo-optic properties of polymer resins can also be determined when there are changes in the refractive index due to temperature changes.8 This thermo-optic effect of polymers combined with their tenability and processing versatility indicates that they can be used as thermo-switches, which can provide additional commercial value for products in the telecommunication industry.9 Optical waveguide devices using organic polymers, such as electro-optic modulators, optical switches, and arrayed waveguide grating, have attracted attention for telecommunication applications due to their simple fabrication, low dispersion, and easily controlled refractive index.10

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Soybean oil is the most readily available and one of the lowest-cost vegetable oils in the world. Epoxidized soybean oil (ESO) is the result of the oxidation of soybean oil with hydrogen peroxide, and either acetic or formic acid. ESO is industrially available in large volumes at a relatively low cost. Due to its low cost and biodegradability over traditional phthalate plasticizers, ESO is replacing dioctyl phthalate (DOP) in some applications. There is continuing demand for its use in industrial applications, such as ESO, soybean modified polyester, etc. ESO is mainly used as plasticizer stabilizer, diluent, and also as a potential green resin to replace epoxy, which shows excellent promise as inexpensive, abundant, and renewable material.11,12

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Materials

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The ESO used in this study contents oxirane within the range 7.1 to 7.2 (EdenolD81; Cognis, Germany). Acrylic acid, hydroquinone, diethyl ether, and calcium chloride were purchased from Sigma-Aldrich (USA), and were used without further purification. Photoinitiators, namely Irgacure, were supplied by Ciba Geigy. The UV absorber HALS (hindered amine light stabilizers; Ciba Geigy), which absorbs at 430 nm, was used together (for system ESO and AESO) with an UV absorber to prevent UV degradation upon aging.

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Modification of epoxidized soybean oils

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Modified ESO were manufactured by heating acids and epoxidized oil; the temperature was maintained at 80 °C for several hours. After the reaction was completed, the crude product was filtered and repeatedly washed with distilled water and diethyl ether. Finally, the oil phase was dried with anhydrous sodium sulphate. Coating formulations with photoinitiators, Irgacure, were prepared at least 1 day before curing and were stored in a cool, dry place.

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Sample preparations

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High-quality glass substrates were washed first with acetone and methanol before spin-coating with a spin coater at speeds up to 5000 rpm. An optimized spin speed of 5000 rpm was used. The resin was prebaked for 5 min at 90 °C before exposing it using an IST UV exposure machine from Fusion UV Systems Inc., Gaithersurg, MD, USA. The dosage of UV radiation used was a current of 7.5 A with an intensity of 0.107W/cm2 and 2m/min conveyer speed, which was measured with a SAIRON SPA 4000Prism coupler, an UV VIS spectrometer by Perkin Elmer Lambda 35. The Vickers hardness and elastic modulus were determined using Shimadzu Dynamic Ultra Micro Hardness Tester DUH-211/DUH-211S (Shimadzu Corporation, Tokyo, Japan).

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Measurement

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UV VIS spectroscopy

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UV spectroscopy of cured epoxidized oil was used to determine the resin transmission property at different wavelengths from 400 to 800 nm for the formulated system employing photoinitiators A and B. All samples exhibited high optical transparency at more than 80%. Transmission was determined with and without the UV absorber. After exposure for 3 and 6 h, the resin transmission decreased. Optical clarity was lost, but transmission was maintained when an UV absorber was added to the system.

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Prism coupler

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A SAIRON SPA 4000 prism coupler with a high refractive index (n) was measured to be 1630 nm. The technique consisted of measuring the angles at which the prism coupled the light from a laser beam onto the sample film. The optical measurements were performed using a prism coupler system with incident beam, as shown in Fig. 1. Measurements were made until coupling occurred, as shown by the minimum in the output from the photodetector. A polarizer was used to switch between the TE and TM mode measurements.

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Variations in the refractive index and thickness at 1550 nm were evaluated. Very thin films up to 1-nm-thick can be evaluated using the prism coupler technique; the results are then compared with the conventional CTE measurement, which measures samples that are a few millimetres thick using the thermal mechanical analysis technique.

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Dynamic ultra micro hardness tester

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To evaluate the strength of the thin film or surface coated layer, a hardness tester was used to apply a low-test force using the electromagnetic loading method and measured the indentation depth with a differential transformer. The DUH 211S hardness tester enables accurate detection of a sample surface by making use of the characteristics of the electromagnetic loading mechanism. Upon reaching a preset maximum value, the normal load was reduced until partial or complete relaxation occurred. This procedure was performed repetitively; at each stage of the experiment, the indenter was positioned relative to the sample surface. The force was varied from 1.96, 19.6, and 196.1 mN. For each loading/unloading cycle, the applied load value was plotted with respect to the corresponding position of the indenter. To detect a sample surface, the DUH 211S detected changes in the indenter as the speed decreased. The indentation modulus EIT was calculated from the slope of the tangent of the unloading curve using a linear fit to the initial unloading data or a power-law fit:4,5Display Formula

EIT=1−ns21Er−1−ni2Ei,(1)

where Ei is the elastic modulus of the indenter, vi is Poisson’s ratio of the indenter, and is Poisson’s ratio of the tested sample. The reduced modulus,Er, which is calculated from the indentation data, is defined as: Display Formula

Er=p.S2.b.Ap(hc).(2)

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Design and Simulation using Opti-BPM

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A schematic view of the device 1×2 optical power splitter designed by Opti BPM designer version 10.0/2010 used for this study is shown in Fig. 2. Principally, the waveguide is designed to have length of L=600um with core (AESO) and cladding refractive indices of ncore=1.49 and ncladd=nair=1.0, respectively. The latter is modified, as will be discussed later, to obtain a relatively high output power. In addition, the wavelength used was 1.55 um, which is the most promising wavelength for optical communications because of minimal attenuation in the optical fiber. Initially, the branching angles of the two output arms, ⊖1 and ⊖2, are assumed to be not equal to an arbitrary value. Thus, an output power dividing ratio is expected to be achieved. Generally, as previously mentioned, the branching angle should be fixed at a small value to decrease the excess loss resulting from the effective uptapering of the branches and to ensure that the optical splitter is adiabatic (i.e., the local super mode does not couple to higher-ordermodes). The width of the waveguide (w), assuming a 10 um single mode operation, was then changed to investigate its influence on the output power.

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UV VIS Spectroscopy

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For resin employing photoinitiator B (Fig. 3), a similar trend was observed after UV exposure; high transmission persists for all samples with and without an UV absorber for exposed and unexposed samples. The film was found to absorb strongly less than 400 nm. Shift in wavelength was observed after exposure. The formulation with photoinitiator B has a higher cut-off wavelength at which resin starts to absorb UV than photoinitiator A (Fig. 4). Resins were also found to have lower transmission between 80% and 85%. n and k are wavelength dependent parameters. The measured transmission is 80% to 85% from UV visible spectroscopy is for wavelength range of 200 to 800 nm. As for refractive index measurement AESO and ESO has n≈1.46 to 1.5 for prism coupling. There was probably some dispersion due to high absorption of material is mainly composed of C–H (carbon-hydrogen) linkage, which has high absorption near the IR region about 1500 to 1000 nm (13).

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The transmittance of the films measured at near IR from 1100 to 500 nm was higher than 80% in the visible region, which is much better than that observed for other epoxy resins. Transmission is approximately 80% to 90% because the resin used was composed of hydrocarbon type C—H, which has some absorption, especially near the visible and UV regions. The high transparency originates from the reduction of intermolecular interactions caused by the bulky oxirane groups in the main chain and the reduction of the intra and intermolecular charge transfer (CT) interactions. The oxirane group acts as an electron donor, and unsaturation along the glyceride chain and acrylate introduces an electron acceptor for charge transfer. The cut-off for both AESOs shown in Fig 4 is approximately 290 nm. AESO with different photoinitiators exhibited different transmittances despite having almost identical cut-off values. The high transmittance of AESO can be ascribed to the reduction of CT interactions in this epoxidized oil because the CT absorptions are generally observed in a longer wavelength region than the cut-off. Factors contributing to the propagation loss of the optical waveguides, such as scattering, and a process-induced effect in addition to the intrinsic absorption loss of the material itself, are caused by the presence of impurities in the solution, the uniformity of the spin-coating step, and the heat treatment of the film. Both the absorption loss and process-induced scattering loss are very low compared with the total loss. Thus, the major part of the measured loss at 1550 nm should be attributed to the loss induced by impurities in the solution. Therefore, by optimizing the filtering process, the propagation loss could be further reduced.14,15

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The absorption decreased with a decreasing number of C–H bonds in the material. However, the absorption was still significant even with a low proportion of C–H bonds. The scattering component can be divided into intrinsic and extrinsic contributions. The latter was caused by external contamination, such as dust and surface roughness, and is assumed the same for each film. The former results from structural in homogeneities, which could originate from the polymerization, processing, and cross-linking steps. However, polymerization was mediated by free-radical reactions and resulted in highly amorphous polymers, as confirmed by the measurement of low birefringence (Δn=nTE−nTM<0.0005). Therefore, the structural in homogeneities were believed to be caused primarily by the shrinkage induced during the cross-linking reaction.16–18

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Prism Coupler

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Measurement at room temperature

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The refractive index measured in the TE and TM modes from the prism coupler at 1550 nm (only at TE) at room temperature for both unacrylated and acrylated thin films is shown in Table 1. The birefringences at room temperature were found to be smaller for unacrylated films than acrylated films. The refractive index and thickness were found to be slightly higher for the TE mode at 1550 nm than the TM mode. A multi-mode profile was obtained for the acrylated for the 1550 nm TE mode and the 1550 nm TM mode at room temperature, as measured from the prism coupler in Fig. 5. Epoxidized oil produces a multi-mode optical resin that is suitable for various optical components for application in short-distance communication systems, such as optical interconnects and local area networks (LANs).

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To obtain polymers with appropriate refractive indices as shown in Table 1, we carefully measured the refractive indices of various polymer materials by using a prism coupling method at a wavelength of 1.55 μm. The acrylated AESO had a higher refractive index for TE polarization than that for TM polarization, and it has much larger birefringence than that of unacrylated. Hence, we exploited the acrylated to determine the polarization of the guided light depending on the shape of the acrylated AESO layer. That large birefringence exhibited by AESO showed that the effective index of the waveguide core was increased for the TE polarization, but decreased for TM polarization. Hence, the TE mode was guided by the waveguide, whereas the TM mode can be radiated out into the core layer.

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The in-plane (nTE) and out-of-plane (nTM) refractive indices of the AESO measured at 633 nm range from 1.7137 to 1.7402 and 1.7041 to −1.7318, respectively. The fact that the values were slightly higher than the nTM for all of the PI films reflects the preferential chain orientation parallel to the film plane. For prism coupling, n (refractive index) and t (thickness) were measured at different spots on each sample. The thickness differences were more than 100% for unacrylated ESO between the TE and TM modes, and less than 20% for AESO. The large difference in the unacrylated ESO sample was due to the intrinsic ESO, which could not easily form a thin film, as it is softer than the acrylated ESO samples as observed from hardness testing. Other factors that can affect the thickness are nonuniformity of the spin-coating step and heat treatment of the film. Thicker film values could have been caused by an edge effect upon spin coating.

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Dynamic Ultra Micro Hardness Tester

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Test force and indentation depth during the indentation process was carried out using the ultra micro hardness tester as measured, in accordance with ISO 14577-1 (Annex A), had been used to evaluate hardness, elastic modulus, and amount of work done.

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The hardness evaluation is very important in the choice of resin material, which can withstand mechanical impact forces due to identity preventing waveguide premature failure and breakage during operation.

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It was found that acrylated AESO has a higher HMV (Vickers hardness) by more than 50%, but the Eit (indentation elastic modulus) value is slightly higher for unacrylated film when a higher indentation force was used, as shown in Fig. 6 and Table 2. HMV and Eit values were found to increase as the test force increased (Fig. 7 and Table 3). The HMV was higher for acrylated AESO when acrylate was used because it reduced the mobility of the alkyl chain of the oil and resulted in greater brittleness. This result can be evaluated further at the higher glass transition temperature exhibited by acrylated AESO. Reduced cohesive forces between chains exist, which increase the polymer chain segmental mobility, thereby reducing its hardness.19 Elastic modulus represents a linear elastic stress strain relationship with no specific direction, unlike Young’s Modulus, which is directional.20

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Insertion Loss Using Opti-BPM

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Beam propagation method (BPM) software was used to obtain the desired output. Figures 8(a) and 8(b) present the simulation results for the normalized output power of the propagating optical beam inside of the proposed structure for ncore(AESO)=1.49, ncladd(air)=1.0, and waveguide width=10μm. Generally, it can be noted that the output power decreased as the length of the waveguide increased due to an increase in transmission loss. The output power was improved further, after the branching region, when the refractive index of the cladding layer was changed to ncladd(air)=1.0, as shown in Fig. 8(a). The corresponding topographical map of the optical field, which was basically the electric field (E) in the electromagnetic waves derived by Maxwell’s equations, is presented in Fig. 8(b). According to this map, the simulated measurements reveal that the output power for each arm is as follows:

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The simulation results indicate good performance for this structure. Table 4 shows the insertion loss (IL) of the optical splitter. The IL of a device is the portion of power that is lost and must be low for good performance.

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The general relationship between the output power of arm B and arm C with the branching angle θ1 is depicted in Fig. 9. From Fig. 9, we observe that the output power of each branch can be controlled by adjusting the corresponding angle. This is of high importance when considering an optical power splitter in optical networks.

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Conclusions

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It was observed that, upon UV exposure, high transmission still persisted for all samples with and without an UV absorber. The film was found to absorb strongly less than 400 nm. A change in the cut-off wavelength was observed upon exposure. Birefringences behave differently between EO and AESO, where it acts as negative birefringence and positive birefringence, respectively, at room temperature.

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The TE and TM polarization refractive indices of AESO films at 632.8 nm were obtained experimentally using a prism coupler. For AESO, the measured TE and TM refractive indices were 1.4849 and 1.4602, respectively. The birefringence of approximately 0.02 is consistent with the amorphous nature of the cross-linked polymer structure. Positive birefringence was obtained by the AESO resin. The wavelength of the emitted light shifts higher in positively birefringent crystals because the slow ray of the crystal is parallel to the slow axis of the compensator, whereas for negatively birefringent crystals, the wavelength of emitted light shifts lower because the fast ray of the crystal is parallel to the slow axis of the compensator (www.wikipedia.org).

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The refractive index and thickness were found to be slightly higher for the TE mode at 1550 nm than the TM mode, and a multi-mode profile was obtained for the acrylated AESO. Acylated AESO also has a higher HMV compared with the unacrylated ESO. Resin is suitable for use in optical devices because it has high transmission with a refractive index (n) between 1.46 for the TE mode, and 1.49 for the TE mode with multi-mode waveguide characteristics.

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AESO as polymer waveguide with a high birefringent was described. The AESO had a higher refractive index for the TE polarization than that for the TM polarization. Guided mode polarization can be exploited by using AESO in optical telecommunication devices. The TE and TM polarization refractive indices of the PPSQ films at 632.8 nm were obtained experimentally using a prism coupler. The measured TE and TM refractive indices were 1.554 and 1.557, respectively. The small birefringence (0.19%) is consistent with the amorphous nature of the cross-linked polymer structure. The optical anisotropy of the monomers was arranged in a certain direction in the polymer, i.e., the alignment forces the polymer materials to generate birefringence. The birefringence causes serious problems in the application of polarization. Lower birefringence is preferred in optical devices for communication, as higher birefringence is thought to increase the optical loss and reduce the performance of the optical system; hence, PGII may be the resin of choice for high-temperature operation. The small difference is thought to originate from the difference between the bond angles of the groups, leading to a more random molecular configuration with the acrylate esters.

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We simulated the performance of a 1×2 optical splitter using the BPM method and a BPM-CAD waveguide optics modeling software system (Optiwave Corporation, ON, Canada K2E7X1). The influence of the waveguide width and branching angle on the output power of the optical splitter using the refractive index of AESO, n=1.49, were investigated. The following remarks can be concluded from this paper:

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Acknowledgments

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This work was sponsored by the National University Malaysia-Universiti Kebangsaan Malaysia (UKM) through grant Nos. UKM-AP-ICT-17-2009 and 01-01-02-SF0493. We thank the project leader for his kindness, valuable assistance, and financial support. We also thank the anonymous reviewers for comments that contributed to upgrading this work.

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Mohammad Syuhaimi Ab. Rahman began his career at Universiti Kebangsaan Malaysia (UKM), in mid-2007 as a lecturer and appointed as a senior lecturer in early 2008, as well as a programme coordinator for the Bachelor of Microelectronic Engineering. In January 2010, he was appointed as Associate Professor in the Department of Electrical Engineering, Electronics and Systems. His research is in the area of optical communications system and Photonic Technology and currently he is leading Spectrum Technology Research Group at UKM, Malaysia.

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Khaled Mohamed Shaktur received the BSc degree in electronic engineering from the Engineering Academy Tajoura, in 1993, and MSc degree in telecommunication engineering from Technical University Brno, Czech Republic, in 2002. He is currently working toward the PhD degree in optical communication engineering at National University Malaysia. His research interests include fabrication of optical waveguide and optical properties of materials.

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Rahmah Mohamed currently serves as a senior lecturer for more than 23 years at Faculty of Applied Sciences UiTM Shah Alam, Selangor. She obtained her PhD in polymer photonicsat Elect, Electronic, and System Engineering, UKM in 2005. Her basic degree is in polymer chemistry and technology, and her master’s degree from Loughborough University of Technology (LUT) UK, in 1994. She is also a paint inspector with professional certification from Tasmania Institute of Technology, Australia in 1992. Her specializations are photosensitive polymer for photonics/dental/coatings, synthesis for dye doped optical polymer waveguide, polymer characterisation, sustainable resin synthesis, polymer materials (degradable plastic and composite), adhesive and coatings, organic fiber-filled thermoplastic and thermoset composites. Her research interest is optical polymer use and integration of polymer in electronic and photonic devices, degradable plastic, polymer composites, and synthesis of green and specialty polymers.

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Wan Aimi Zalikha Ahmad Sidek is currently studying as final year postgraduate at Faculty of Applied Sciences UiTM Shah Alam, Selangor. She obtained her BSc degree in material technology from UniversitiTeknologi MARA (UiTM) Shah Alam, in 2009. Her research interest is characterization and application of coating from green sustainable oil and resin. She has presented in several material conferences. She is a student member of Plastic and Rubber Institute Malaysia.

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Norwimie Bt Nawiis completing her postgraduate studies at Faculty of Applied Sciences UiTM Shah Alam, Selangor. She obtained her BSc degree in material technology from UniversitiTeknologi MARA (UiTM) Shah Alam, in 2008. Her research interest is synthesis, characterization, and application of coating from green sustainable oil and resin. She has presented in several material conferences and published some journals on material. She had undergone industrial training program at Toray BASF PBT Resin SdnBhd in Kuantan, Malaysia in 2007. She is a student member of Plastic and Rubber Institute Malaysia.

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